Methods for evaluating projection datasets of an object undergoing examination and the methods for acquiring such projection datasets are generally known. One-dimensional or two-dimensional projection datasets of the object undergoing examination are acquired by means of the acquisition methods. A two-dimensional or three-dimensional reconstruction of the object undergoing examination is as a rule determined by means of the evaluating methods. The reconstruction is evaluated further. Evaluating can be performed automatically by a computer. It is alternatively or additionally possible for (in the case of a two-dimensional reconstruction) the reconstruction itself or, as the case may be (in the case of a three-dimensional reconstruction), two-dimensional representations of the reconstruction to be fed out to a user (usually a doctor) on a display device.
The acquisition methods and the corresponding evaluating methods are differentiated according to multifarious criteria. The kind of signal carrier, for example ultrasound, magnetic resonance, or X-rays, can be cited as the first differentiating criterion.
The acquisition methods and the evaluating methods are further differentiated according to the nature of their subsequent evaluating. For example the object undergoing examination is to be reconstructed using the projection datasets. This limitation usually requires the object undergoing examination to within the scope of the acquisition method be located within the range of a swiveling axis, an X-ray source to be swiveled around the swiveling axis, an X-ray detector to be swiveled correspondingly while the X-ray source is being swiveled so that the X-ray detector will at any time be located diametrically opposite the X-ray source with reference to the swiveling axis, and in each case one of the projection datasets to be recorded through appropriately actuating the X-ray source and X-ray detector at swiveling angles of the X-ray source and stored. The respective swiveling angle is assigned to the respective projection dataset. A recording instant at which the respective projection dataset was recorded is generally also assigned to the respective projection dataset. The angular range through which the X-ray source is swiveled is as a rule greater than 180°. A typical angular range is, for example, 200°, 220°, or 270°.
The acquisition methods and corresponding evaluating methods are further differentiated according to the nature of the object undergoing examination. For instance there are objects undergoing examination that are static. In that case it will suffice for the X-ray source to execute a single swiveling action around the swiveling axis. The time required for swiveling is not critical. The projection datasets can in the case of said kind of individual embodiments be acquired by means of, for instance, a CT system or C-arm X-ray system.
It is alternatively possible for the object undergoing examination to move, in particular iteratively. A typical instance of an iteratively moving object undergoing examination is the human heart. A CT system is as a rule used for performing the acquisition method in the case of said type of objects undergoing examination. The reason is that in CT systems the X-ray source rotates around the swiveling axis at a rotational speed of 75 rev/min and more. Rotational speeds of 120 to 180 rev/min are even possible with CT systems of more modern design. So the X-ray source rotates much faster in CT systems than in C-arm systems, which—depending on the specific embodiment—require at least three seconds for the X-ray source to swivel once through approximately 200° to 220°. The angular range (180° or more) required for determining the reconstruction can therefore be traversed in a much shorter time using a CT scanner than when a C-arm system is used.
Described in the older German patent application 10 2005 016 472.2 is a possibility based on which a C-arm X-ray system can be employed for acquiring the projection datasets of the object undergoing examination although the object undergoing examination moves iteratively. Said patent application was still unpublished prior to the application date of the present invention. It therefore does not constitute a general state of the art but simply has to be considered as part of the examination for novelty within the German patenting process.
A plurality of swiveling actions are performed according to the disclosure in DE 10 2005 016 472.2. With reference to each recorded projection dataset, phase information about the object undergoing examination is additionally recorded and assigned to the respective projection dataset.
It is furthermore possible for the object undergoing examination neither to be purely static nor to move rapidly iteratively, but nonetheless to change relatively slowly. A typical example of an object of said type undergoing examination is the human brain when an injected contrast medium gradually spreads within the brain. Long time series of projection datasets are recorded in such cases, for example over a period of from 30 seconds to 1 minute.
CT scanners are predominantly employed in the prior art for recording projection datasets of said type. Reference is made by way of example to the technical articles “Noise Reduction by Temporal Estimation in Perision Computed Tomography” by P. Montes and G. Lauritsch, IEEE Nuclear Science Symposium Conference Record, Oct. 23-29, 2005, Wyndham El Conquistador Resort, Puerto Rico, and “Analysis of Time Resolution in Dynamic Computed Tomography for Perfusion Studies”, also by P. Montes and G. Lauritsch, IEEE Nuclear Science Symposium Conference Record, Oct. 16-22, 2004, Rome, Italy.
With the last-cited procedure a sequence of reconstructions is determined using the time series of projection datasets. Important physiological parameters relating to the supply of blood to human tissue can be determined using the sequence of reconstructions. Examples of such parameters are the volume of blood, blood flow, permeability etc. The methods have recently become established and are being applied in practice. They are, though, owing to the spatial limitations of CT systems restricted to the diagnostic sphere. They cannot be used in the interventional sphere.
C-arm X-ray systems allow far better access to the object undergoing examination. They are therefore employed not only in the diagnostic sphere but also in the interventional sphere. However, they are as a rule used only for image recording as such.
A method for evaluating datasets of an object undergoing examination is known from WO 2006/003578 A1. Each projection dataset is assigned a recording instant at which the respective projection dataset was recorded by an X-ray detector located diametrically opposite an X-ray source with reference to a swiveling axis. Each projection dataset is furthermore assigned a swiveling angle through which the X-ray source had at the recording instant swiveled with reference to the swiveling axis. Together with the swiveling angle assigned to the respective projection dataset, each data element of each projection dataset defines a projection line along which an X-ray beam has traveled on its way from the X-ray source to the X-ray detector. The projection datasets are combined by the computer into recording groups. Each recording group extends in temporal terms from a starting instant to a finishing instant. The recording groups overlap in time. The computer determines a three-dimensional reconstruction of the object undergoing examination using the projection datasets of in each case one recording group. The respective reconstruction can refer to an instant corresponding with the mean between the starting instant and finishing instant.